This invention relates to a solid-state image pickup device for use in picking up image and, more particularly, to a CCD solid-state image pickup device of interline transfer (ILT) type.
A conventional solid-state image pickup device comprises a plurality of photoelectric converting sections, arranged in the configuration of a matrix with m rows and n columns where m and n represent first and second positive integers each of which is not less than two, n vertical charge transfer sections adjacent to the n columns of the photoelectric converting sections at one side thereof, each of the n vertical charge transfer sections extending along a vertical direction, a horizontal charge transfer section connected to one ends of said n vertical charge transfer sections and extending along the horizontal direction, a shading film for shading first areas between the photoelectric converting sections adjacent in a horizontal direction to each other or second areas between the photoelectric converting sections adjacent in a vertical direction to each other.
The photoelectric converting sections accumulate, in response to incident light, electric charges as signal charges. The n vertical charge transfer sections are for reading the signal charges out of the photoelectric converting sections as read charges to transfer the read charges along the vertical direction as vertical transferred charges. Each of the n vertical charge transfer sections includes m clusters of vertical charge transfer electrodes alternately arranged along the vertical direction and a vertical charge transfer region formed beneath the m clusters of vertical charge transfer electrodes The horizontal charge transfer section receives the vertical transferred charges from the n vertical charge transfer sections horizontal line by horizontal line as received charges to transfer the received charges along the horizontal direction as horizontal transferred charges. The device further comprises an output circuit section which is connected to an end of the horizontal charge transfer section. The output circuit section receives the horizontal transferred charges to convert the horizontal transferred charges into a voltage signal. Those components are isolated from one another by a plurality of element isolation sections.
In one example of the conventional solid-state image pickup device, the n vertical charge transfer sections have a plurality of first, second, third vertical charge transfer electrodes and charge transfer channels. Above the channels of the n vertical charge transfer sections, a plurality of the first and the second vertical charge transfer electrodes are formed to extend in the horizontal direction of an image pickup region. The third vertical charge transfer electrode is also formed to extend in the horizontal direction of the image pickup region, adjacent to the horizontal charge transfer section. Accordingly, the third vertical charge transfer electrode can also be called, "the final vertical charge transfer electrode". Except for each opened area above each of the photoelectric converting sections, a shading film is formed on the image pickup region and the horizontal charge transfer section. An end of the channel of each of the n vertical charge transfer sections is connected to a channel of the horizontal charge transfer section. Above the channel of the horizontal charge transfer section, a plurality of primary and secondary horizontal charge transfer electrodes are formed. Each of the primary and the secondary horizontal charge transfer electrodes extends over an area where a thick SiO.sub.2 film is formed (Hereinunder, called a field area) to be connected to a horizontal bus line through a contact hole.
In the first conventional solid-state image pickup device mentioned above, the vertical charge transfer pulses are supplied from both ends of the image pickup region to the first, the second, and the third (final) vertical charge transfer electrodes. As a result, bluntness of waveform of the vertical charge transfer pulses is caused to occur in a central portion of the image pickup region due to resistance and capacitance of the first, the second, and the third (final) vertical charge transfer electrodes. This brings about deterioration of such a vertical charge transfer efficiency. The problem is serious when the solid-state image pickup device has a large effective picture elements area in size, for example, an optical size of one inch.
In order to overcome this problem, proposal has been made by Unexamined Japanese patent publication 216672/1992. In this paper, a method of preventing the aforesaid bluntness of waveform of the pulses is disclosed. In this method, charge transfer pulses are supplied to vertical charge transfer electrodes by metal wires formed along each vertical charge transfer section and thereabove.
In the second conventional solid-state image pickup device, the first, the second, and the third (final) vertical charge transfer electrodes are connected, through lining contacts, respectively, to power feeding wires each of which extends in a vertical direction along each vertical charge transfer section. The power feeding wires are made of tungsten, aluminum, or the like, and serve as first shading films for the n vertical charge transfer sections. The power feeding wires are isolated from each other between picture elements adjacent in a horizontal direction. As a result, the power feeding wires are not capable of shading areas (Hereinunder, called ferry portions) between the photoelectric converting sections adjacent in a vertical direction to each other. Accordingly, it is inevitably caused to occur that smear is increased. In order to shade the ferry portions, second shading films are formed in a horizontal direction in addition to the power feeding wires serving as the aforesaid first shading films. Furthermore, a third shading film made of aluminum, and the like, is formed over the areas from the horizontal charge transfer section to the photoelectric converting sections.
In the interim, when deterioration is caused to occur in charge transfer efficiency from the vertical charge transfer sections to the horizontal charge transfer section, a fixed pattern noise is generated in a regenerated image. The fixed pattern noise is observed as a fixed pattern like a black line extending in a vertical direction in the regenerated image, which makes quality of image be quite deteriorated.
In the second conventional solid-state image pickup device, like the first and the second vertical charge transfer electrodes, the third (final) vertical charge transfer electrode is also connected to one of the power feeding wires through the lining contact. As a result, bluntness of waveform of the vertical charge transfer pulses is small in size, if caused to occur, in a central portion of the image pickup region. Accordingly, compared with the first conventional solid-state image pickup device, vertical charge transfer efficiency is improved to prevent the fixed pattern noise from being generated.
It is a recent trend that solid-state image pickup devices have relatively small optical size of, for example, one-third inch or one-fourth inch. In case that such a solid-state image pickup device is designed to have, for example, four hundreds thousands effective picture elements, the picture element is forced to have 5.times.5 through 8.times.8 micron meters in size. When the picture element has such a small size, smear characteristic becomes serious as one of main characteristics of a solid-state image pickup device. The smear characteristic degrades because spurious signal charges flowing the vertical charge transfer sections increase. The smear is observed as a noise like a white line in a regenerated image.
One of the reasons of the smear is that incident light enters through chink between a shading film and a surface of a substrate. Namely, the incident light reaches the channels of the vertical charge transfer sections and be subjected to photoelectric conversion therein. The photoelectric conversion is caused to occur due to multi-reflection between the surface of the substrate and the shading film or the vertical charge transfer electrode.
In the second conventional solid-state image pickup device, the second shading films are formed on the above-mentioned ferry portions to improve the smear characteristic More incident light enters into the vertical charge transfer sections through the ferry portions in the second conventional solid-state image pickup device than in the first conventional solid-state image pickup device. The reason is that a distance between the second shading films and the surface of the substrate in the second conventional solid-state image pickup device is larger than another distance between the shading film and the surface of the substrate in the first conventional solid-state image pickup device. This is further because an insulation film exists between the power feeding wires and the second shading films in the second conventional solid-state image pickup device.
Thus, the second conventional solid-state image pickup device has a problem that the smear characteristic is not sufficiently improved.
In order to improve the smear characteristic sufficiently, it can be considered that a distance between adjacent power feeding wires are designed to a narrow one, such as 0.1 through 0.2 micron meters only above the ferry portions. Patterning of such power feeding wires requires a high-technology, such as direct electron beam lithography of photo resist. Introduction of such a new process technique brings about some disadvantages, such as changes of manufacturing process, increase of manufacturing cost, and the like. Further, leavings of etching are easily produced in the ferry portions, because the ferry portions have large steps due to overlapping of the vertical charge transfer electrodes. It is therefore difficult that the adjacent power feeding wires are formed to have a narrow distance, such as 0.1 through 0.2 micron meters only over the ferry portions.
On the other hand, in the first conventional solid-state image pickup device, it can be considered that the vertical charge transfer pulses are supplied through an independent terminal only to the third (final) vertical charge transfer electrode. Thereby, load against the vertical charge transfer pulses becomes small, bluntness of waveform thereof can be prevented. However, this causes another undesirable problem that one terminal is further required for the independent terminal